The effective extension of the communication distance and the data capacity in next-generation optical signal technology is no longer a theoretical possibility hence becoming the feasible solutions for the modem optical network. This is due, mainly, to the combination of several critical technological advancements, such as, 1) coherent optical detection development, 2) multi-level (differential) phase-shift-keying (DPSK) modulation format adoption, 3) progress in adaptive electrical equalization technology.
FIG. 1 shows a schematic of a quadrature modulator 1. The principle of its operation is as follows. Input optical beam 2 is splitted into two arms of the MZ interferometer. Two Mach-Zehnder modulators (4 and 5) are placed in parallel; each MZM being located in each arm of the MZ interferometer. The biases of the MZMs are controlled by Bias 1 and Bias 2 (6 and 7) and driven by RF1 and RF2 data signals (8 and 9). The Phase port 10 of the QM controls relative phase shift between the arms of the MZ interferometer. An output beam 11 can be transmitted via an optical link.
On the receiving side a coherent detection is implemented to decode the received optical beam. In contrast to existing optical direct-detection system technology, an optical coherent detection scheme would detect an optical signal's amplitude as well as its phase and polarization. Within the fixed optical bandwidth more data can be transmitted using a coherent detection scheme with increased detection capability and spectral efficiency. Coherent detection provides increased receiver sensitivity by 2-6 dB compared to an incoherent system. In addition, since coherent detection enables an optical signal's phase and polarization to be measured and processed, the transmission impairments that previously presented challenges to accurate data reception can, in principle, be mitigated electronically when an optical signal is converted into the electronic domain. Tier-1 network providers have now realized the potential for optical coherent systems by deploying DPSK systems with improved DSP receiving circuits based on complicated optical phase-lock loops.
The optical hybrid (FIG. 2) is the critical part of the coherent receiver 20 needed to combine a local oscillator wave, Lo, with the received signal, S. Such an optical hybrid 21 is a key component in phase- or polarization-diversity schemes. Ideally, the hybrid should combine waves with quadrature relative phases at the outputs, providing the advantage of base-band processing. In a two-phase case outputs must be mutually phased at 90° (in-phase and quadrature, referred to as I and Q signals). In FIG. 2 couplers 22-25 are used for the waveguides combining and splitting, the phase shifter 26 introduces 90-degrees phase shift between I and Q components.
If the single phase shifter (or alternatively) two phase shifters provide a 90-degrees phase shift, then all four output beams received by a set of balanced photodetectors have 90-degrees relative phase difference of the form:{A=S+Lo,B=S−Lo,C=S+jLo,D=S−jLo}.
The signals from the detectors 27 and 28 are processed in a digital signal processing unit and may be further displayed or used for continuing processing.
The coherent receiver may operate in homodyne or self-homodyne regime. In the self-homodyne implementation, the inputs are the received signal S and a delayed replica of it delayed by one symbol. In homodyne implementation, the incoming beam is mixed with a local oscillator beam from a local oscillator (not shown in the figure).
The goal of this invention is to use coherent transmitters/receivers, and channel compensation algorithms to achieve agile free-space optical communications links. The flexible architecture will provide secure, robust, multi-rate/multi-format optical transmission that is resistant to jamming and eavesdropping and achieves spectrally-efficient high data rate throughput in any challenging communication environment.